In recent years, in the field of life sciences, it is becoming highly important to produce an object useful protein in an industrial scale by mass culture of an animal cell that produces the useful protein. When a natural or recombinant protein and the like are produced by culturing the animal cell or an animal cell introduced with a gene encoding a desired protein, an animal cell growth factor is used besides the basic nutrients, such as salts, saccharides, amino acids, vitamins and the like, for the growth of the animal cell. As the animal cell growth factor, serum components such as fetal bovine serum, calf serum and the like are generally used. In general, when fetal bovine serum or calf serum is used, it needs to be added to a basal medium in about 5-20 volume %.
However, sera such as fetal bovine serum, calf serum and the like are in limited supply and are generally highly expensive. This leads to an increased production cost of the useful protein.
Moreover, mammal-derived growth factors and sera are problematic since they are related to mad cow disease, Bovine Spongiform Encephalopathy, transmissible spongiform encephalopathy, and further, prion-related diseases such as Creutzfeldt-Jakob disease, and mammal-derived serum may be contaminated with virus or mycoplasma since it cannot be sterilized in autoclave and the like. From the aspect of safety, therefore, a serum-free culture method has been developed wherein animal cells are cultivated in the absence of serum. In the serum-free culture method, naturally occurring purified proteins such as insulin, transferrin, growth factor and the like, recombinant proteins and the like are used as a substance that substitutes the effect of serum (serum alternative). However, these proteins are generally more expensive than fetal bovine serum or calf serum, and the supply thereof is limited.
As mentioned above, since serum and serum alternatives are expensive, and the supply thereof is limited, the development of a technique for producing useful proteins still more efficiently is desired.